The present invention relates to a system for controlling fuel injection in an internal combustion engine that has a cylinder injector for directly injecting fuel into a combustion chamber defined inside a cylinder, and a port injector for injecting fuel into an air-intake passage.
Conventionally, fuel injection into the intake passage, such as an intake port (hereinafter referred to as “port injection”), has been widely used for supplying fuel into the combustion chamber. In the port injection, the fuel is injected into a passage upstream of the intake valve that is disposed at the entrance of the cylinder. The fuel injected in this manner is introduced into the combustion chamber during the suction stroke of the cylinder in a mixed state substantially uniformly with the air.
On the other hand as an alternative injection method, direct injection of the fuel into the cylinder (hereinafter referred to as “cylinder injection”) has recently been suggested. In the cylinder injection, the fuel injection pressure is set comparatively higher than that in the case of using port injection to inject the fuel in an atomized state. The atomized fuel can be easily vaporized. In the cylinder injection, the temperature within the combustion chamber can be reduced by the vaporization heat as the vaporization of the atomized fuel occurs. Because the temperature inside the cylinder is reduced, the suction efficiency is improved and results in the increase in engine output.
In the cylinder injection method, deposits due to soot or particulate matter after fuel combustion can be accumulated in the vicinity of the fuel injection orifice because the tip of the injector is exposed within the combustion chamber. The accumulation of the deposits can decrease the amount of fuel injected and can change the injection conditions after time so that the combustion state can be deteriorated.
Accordingly, as described in Japanese Patent Laid-Open Application 63-138120, the injection method can be compulsorily transferred from the cylinder injection to the port injection after a predetermined period though the engine is in operation under conditions that enable cylinder injection. By forcibly switching the combustion injection method in this way, the temperature at the tip of the cylinder injector is increased to enable periodical burning of the accumulated deposits to reduce the accumulation.
However, the periodic transfer between the injection methods cannot perform appropriate transfer between the injection methods responsive to the amount of the accumulated deposits because the changeover can take place even when the deposits have not yet actually accumulated. Further, because the cylinder injection is discontinued when totally switching the injection method to the port injection, the advantageous effect of cylinder injection, such as improved suction efficiency due to reduced chamber temperature, cannot be obtained.
In addition, the disadvantages such as decrease in the fuel injection amount and the change in the form of atomized vapor are not always caused by deposit accumulation. Rather, such disadvantageous effects can also be caused by other factors, for example the decrease in the fuel injection pressure by some malfunction of the injector.